It is necessary for the water tubes of vertical steam generators used in nuclear power plants to be periodically checked. In this respect, the water constituting the primary fluid and contaminated by the nuclear reactions must not be able to pass into the secondary fluid which is used for operating a turbo-alternator. This check is made with the aid of a Foucault current probe which is passed through each of the tubes. A slit or hole in the tube or even a simple reduction in its thickness is then converted into a variation in the output current from the probe.
In practice, the probe is carried by an apparatus mounted movably in the lower chamber of the generator, into which all the tubes open. This apparatus carries a guide tube which is brought in front of a tube of the bundle and into which the probe is inserted. After checking a tube, the probe is withdrawn from the tube, the apparatus is moved through a distance equal to the distance between two tubes, and the probe is inserted into a new tube. These various operations are controlled from the outside because as the chamber wall has been in contact with irradiated water, it itself emits ionizing radiation. The probe-holding apparatus proposed hitherto comprises expansible mandrels which are inserted and fixed into the tubes of the bundle. However, with such apparatus, it is often difficult to check those tubes disposed at the bundle periphery. Moreover, any given apparatus is suitable only for checking tubes which are a given distance apart. If it is required to check the tubes of a steam generator in which the tube pitch is different, the apparatus has to be considerably modified. Finally, such apparatus is large and bulky.